Extended release formulations for oral administration of drugs are preferred for a number of reasons. For example, they enable the patient to ingest the formulation less frequently, which may lead to increased patient compliance with the dosing regimen. They may also result in fewer side effects, as peaks and troughs of the level of the drug in the bloodstream of the patient may both be decreased, leading to a more even drug level in the blood over a period of time. Such formulations may also provide a longer plateau concentration of the drug in the blood.
Many different types of extended release formulations are known in the art. Currently, sustained and controlled release drug delivery systems administered by the oral route are usually based on either a gel forming matrix or coated formulations, or the combination thereof. The selection of the proper type of such an extended release formulation is crucial for effective drug delivery which minimizes side effects, and hence for patient compliance.
Venlafaxine, 1-[(2-dimethylamino)-1-(4-methoxyphenyl) ethyl]cyclohexanol, is an important drug in the neuropharmacological arsenal, used for the treatment of depression. Venlafaxine and the acid addition salts thereof are disclosed in U.S. Pat. No. 4,535,186, which is hereby incorporated by reference as if fully set forth herein. Venlafaxine hydrochloride may be administered to adults in compressed tablet form in doses ranging from 75 to 350 mg/day, in divided doses two or three times a day. It has been found that in therapeutic dosing with venlafaxine hydrochloride tablets, rapid dissolution results in a rapid increase in blood plasma levels of the active compound shortly after administration followed by a decrease in blood plasma levels over several hours as the active compound is eliminated or metabolized, until subtherapeutic plasma levels are approached after about twelve hours following administration, thus requiring additional dosing with the drug. With the plural daily dosing regimen, the most common side effect is nausea, experienced by about forty five percent of patients under treatment with venlafaxine hydrochloride. Vomiting also occurs in about seventeen percent of the patients.
U.S. Pat. No. 6,274,171, issued on Aug. 14, 2001, describes one attempted solution to the problem of frequent administration of venlafaxine. The disclosure teaches an extended release formulation of spheroids, which feature an outer film coating over spheroids containing venlafaxine, and which are placed in a hard gelatine capsule. However, spheroids are a more costly and less efficient solid dosage form to produce, and also require the additional procedural step of being placed in a hard gelatin capsule.
Tablets are a less costly and a more efficient solid dosage form to manufacture, but as taught by U.S. Pat. No. 6,274,171, the production of tablets for an extended release formulation of venlafaxine does not provide satisfactory results (see column 4 lines 60-67, for example), both in terms of physical instability and too rapid dissolution.
Thus, U.S. Pat. No. 6,274,171 teaches that a formulation for venlafaxine of encapsulated spheroids is the only feasible solution for extended release.
Extended release tablet formulations are known in the art. Many of these formulations use combinations of polymers such as HPMC (hydroxypropyl methylcellulose) and ethyl cellulose. For example, U.S. Pat. No. 4,657,757 teaches the use of either HPMC alone, or a combination of HPMC and ethyl cellulose. However, the use of a filler, or of a special release controlling coating, are not taught or suggested.
U.S. Pat. No. 6,274,171 clearly teaches that an extended release formulation of venlafaxine ameliorates sharp peaks in the blood levels, which are not desired, and that in order to avoid side effects and sharp peaks in blood levels, an extended release formulation is required. However, U.S. Pat. No. 6,274,171 teaches that a tablet formulation for extended release of venlafaxine is not feasible.
U.S. Pat. No. 4,601,894 teaches that HPMC is preferably used in combination with a filler. However, it does not teach the combination of a water soluble cellulosic polymer, such as HPMC, with a water insoluble cellulosic polymer, such as ethyl cellulose. It also does not teach the use of a special release controlling coating.
U.S. Pat. No. 5,451,409 teaches that a combination of HPMC and HEC (hydroxyethyl cellulose) may be used in a sustained release formulation, but teaches that a filler, such as microcrystalline cellulose, is not suitable in combination with cellulosic polymers for a sustained release formulation. It also does not teach the use of a special release controlling coating.
U.S. Pat. No. 6,217,903 teaches the combination of two or more cellulosic polymers, but specifically teaches away from the combination of HPMC and ethyl cellulose. Also, it teaches the use of relatively low amounts of fillers, such as microcrystalline cellulose for example, of less than 30%, and typically only about 20% or less as a weight percent of the total formulation. It also does not teach the use of a special release controlling coating.
U.S. Pat. No. 6,350,471 teaches the combination of a water insoluble but water-permeable film forming polymer and a water soluble polymer, with a plasticizer, for forming a delayed release coating. However, the taught cores are immediate-release cores, or at least do not control the release characteristics of the formulation. Furthermore, it does not teach that a controlled release core would be suitable for use with the taught coating, as various deleterious effects (such as capping for example) might be expected to result.